Hi, I've had the same problem and it's probably because you didn't do the 'Safety Removal' thing when you took it out of the computer or card reader (just as you do with a USB data stick). This totally mucks it up and says that there aren't any pictures, but when you go to take a photo, it says memory full (on mine anyway)
Try going to a camera shop and see if they can do anything--i'll put another answer on if i figure it out or if the shop can fix it!
Spike, August 2008